The present invention relates to a current switch for monitoring a current level in an electrical circuit and enabling an alarm if the current level is abnormal and, more particularly, to a current switch that is automatically calibrated to the current level of a circuit when it is installed in the circuit.
Many industrial and commercial environments utilize large numbers of devices that are electrically powered. The operation of these devices, for example fans, pumps, compressors and heaters, can be important to the protection of valuable property and successful completion of processes that may involve costly or hazardous equipment, operations or materials. These devices may be stand-alone devices controlled by a local controller, but are often widely dispersed components of an integrated and, commonly, automated system that are monitored and operated by a remotely located controller or building management computer. The operation of a device or load is commonly monitored by a current sensor that is electromagnetically coupled to a cable supplying electrical power to the load. The current sensor outputs a signal that is representative of the level of current flowing in the cable and, if the current changes significantly, an alarm signal is transmitted to the controller which may display a warning or an advisory signal on a control panel for a human operator and/or selectively enable or disable power to the load and/or to other load(s) that may be effected by a malfunction of the monitored load.
Holce et al., U.S. Pat. No. 5,808,846, incorporated herein by reference, disclose a protection device comprising a combination current sensor for monitoring current in a cable supplying power to a load and a relay controlling the operation of the load in response to a signal from a remotely located control panel. The protection device includes a sensing transformer comprising a wire wound core that encircles the power cable. A changing current in the power cable produces a varying electro-magnetic field around the cable which, in turn, induces a magnetic flux in the core of the sensing transformer. The magnetic flux in the core induces a voltage in the wire windings that is representative of the current flowing in the power cable. Thus, the power cable is the primary winding and the wire winding is the secondary winding of the sensing transformer. The wire winding is electrically connected to an input circuit that converts the voltage signal received from the secondary winding of the sensing transformer to an output signal representative of the current flowing in the power cable. The output signal is transmitted to a control panel and analyzed to determine if the controlled device is to be disabled or enabled. The control panel transmits an appropriate signal to a relay or switch circuit, typically comprising a triac or relay, which responds to the signal from the control panel by shorting or isolating electrical terminals in series with the controlled load.
While electric loads, such as motors, are often operated with electric power from sources that have a constant frequency, for example 50 or 60 hertz (Hz.) for the U.S. electrical distribution grid, variable frequency drives are, increasingly, being used to supply power to AC motors and other loads in residential, industrial and commercial systems. The speed of the motor can be varied by varying the frequency of the output of the variable frequency drive enabling improved control over the fan, pump or other mechanical load powered by the motor and an increase in system energy efficiency. However, both the frequency and the voltage of the variable frequency drive's output vary producing a substantial variation in the current to the motor. False alarms are common when a current switch with a single alarm set point current is used in combination with a variable frequency drive.
Cota et al., U.S. Pat. No. 5,705,989, incorporated herein by reference, disclose a current monitor for a load connected to a variable frequency drive. The current monitor comprises a measuring system with a sensor, comprising a current transformer, linked with a power cable supplying power to an electrical load, such as a motor. The measuring circuit includes a plurality of circuit models each comprising a frequency band, a range of frequencies, and a respective reference voltage or current. In the operating mode, the frequency of the signal in the power cable is sensed by a frequency counter and a frequency band selector determines the stored reference power cable voltage or current that corresponds to the frequency band that includes the frequency of the alternating current detected in the power cable. The measuring circuit compares the selected reference voltage or current to the actual voltage or current detected in the power cable by the sensor to determine if an alarm condition exists. By correlating the frequency of the power cable signal with the magnitude of the power cable current or voltage an abnormal current warranting an alarm can be determined accurately. In a preferred embodiment, the measuring circuit automatically learns an amplitude of the reference voltage or current for each of the different frequency bands when a reset control is activated or upon the expiration of an updating interval. Automatically updating the reference voltages or currents for each frequency band enables the measuring system to adapt to a dynamic electrical system.
However, when a system is initially installed or a new current sensor is added to an existing system, the operating range of the current in the power cable must be determined and the sensor calibrated for the expected range of currents. Calibrating large numbers of widely dispersed current sensors or even one sensor that is remotely located from the controller can be time consuming and tedious. What is desired, therefore, is a current sensor that automatically calibrates itself when installed in an electrical circuit.